An artist's rendition of a galaxy with a supermassive black hole at its core. Such a galaxy is called an Active Galactic Nucleus (AGN) (Credit: NASA)

Episode 2According to the unified model for AGNs, it all depends on your point of view. The central black hole is assumed to be surrounded by a thick donut-shaped cloud of gas and dust. The
source looks different, depending on whether it is observed from the side through the edge of the donut (Type 2), or from the top through the hole (Type 1).

The unified model explains many of the observations of active galaxies, and has won widespread acceptance among astronomers, though questions remain. Chief among these is whether this unified model
applies to quasars whose central black hole is at least a thousand times more energetic than the typical AGN. Could a cloud of gas exist for any length of time around a giant black hole that is
generating energy at such a prodigious rate?

Episode 1
March 20, 2000 :: Could giant black holes be hiding in the centers of galaxies that lead double lives, appearing as mild-mannered "Clark Kent-like" normal galaxies in visible light, but revealing a
powerful "Superman-like" inner core in X-rays? More